When towing in-water equipment behind a vessel, such as when conducting seismic surveys, it is desirable that the towed equipment be positioned at some lateral offset, either to port or to starboard, with respect to the track of the vessel. To position, or divert, the towed equipment to one side of the vessel or the other, a device is typically deployed in the water that is capable of generating hydrodynamic lift in a direction that is perpendicular to the direction of travel of the vessel. A number of such devices are known and are variously referred to as “divertors”, “deflectors”, “paravanes”, “vanes”, and “trawl doors” or simply “doors”. As used herein, the term “divertor” or “divertors” refers to all such devices.
Divertors rely on a wing or foil to generate the necessary hydrodynamic lift. Divertor designs range from the use of a single foil or wing, to as many as 5 or more individual foils arranged laterally in a cascading progression, one behind the other. The foils themselves can be simple flat or bent plates, classical air foils, such as NACA, Eppler, or Gottingen series profiles, or proprietary custom foils. The foils may also have features, such as flaps, slats, slots, and wing tips to help achieve maximum lift coefficients and/or minimum drag coefficients. Foil design and features have a significant impact on the divertor's performance, such as, its lift and drag coefficients. The aspect ratio of the divertor is also an important design parameter.
Aspect ratio is the length, or span, of a wing compared to its chord, which is the distance from the leading edge to the trailing edge of the wing. For very low aspect ratios, such as 1:1 or less, the divertor's lift-to-drag ratio may be in the range of 2:1, which is considered poor. As the aspect ratio is increased above 1:1 the lift-to-drag efficiency improves. For example, an aspect ratio of 2:1 can yield lift-to-drag ratios of 4:1 or higher. By continuing to increase the aspect ratio, lift-to-drag efficiencies can be achieved as high as 10:1.
While high aspect ratio divertors generate high lift for minimal drag, they also present operational challenges in terms of deployment and recovery from the vessel. Divertors require dedicated handling gear for launch, retrieval, and onboard stowage, including cradles, davits, overboarding booms, and structural modifications to the side or stern of the vessel. Skilled operators are required to launch these divertors, especially in rough sea states. Generally, long and narrow divertors tend to be more problematic for handling and stowage than short and wide divertors.
Divertors use a fixed bridle of chain, rope, or wire to launch and tow the divertor. The lengths of the bridle legs and their attachment points determine the angle of attack at which the divertor tows through the water and, consequently, how much lift is generated by the divertor. The bridle configuration is set prior to launch, such that the divertor immediately starts generating lift as soon as it enters the water. This presents operational challenges for the operator during launch and retrieval of the divertor, particularly in rough sea states. Prior to launch, the operator must ensure that the divertor is secure and under control while the vessel is pitching and rolling. The operator must also ensure that the divertor enters the water with an attitude that ensures that it remains stable, starts generating lift in the desired direction, and immediately starts pulling away from the vessel. During the interval while the divertor is entering the water, and in close proximity to the vessel as it starts to generate full lift, high snap loads can be produced in the towing cable as the vessel and divertor are both reacting to rough seas.
In order to manage the high dynamic loads generated during launch, retrieval, and towing in rough seas, the mechanical design of divertors requires strong structural members with a thick cross-section. This results in a heavy divertor that places additional demands on the vessel's handling gear. For those applications where the divertor is intended to operate near the surface, a heavy divertor also requires a large surface reference float, which increases the drag of the divertor, as well the requirements for handling and stowage.